Method for modifying the refractive index of ocular tissues

ABSTRACT

A method for providing vision correction to a patient. The method includes: (a) measuring the degree of vision correction needed by the patient and determining the location and shape of refractive structures that need to be positioned within the cornea to partially correct a patient&#39;s vision; (b) directing and focusing femtosecond laser pulses in the blue spectral region within the cornea at an intensity high enough to change the refractive index of the cornea within a focal region, but not high enough to damage the cornea or to affect cornea tissue outside of the focal region; and (c) scanning the laser pulses across a volume of the cornea or the lens to provide the focal region with refractive structures in the cornea or the lens. Again, the refractive structures are characterized by a change in refractive index, and exhibit little or no scattering loss.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 14/109,542 filed on Dec. 17, 2013, which is a divisional applicationof U.S. application Ser. No. 12/895,978 filed on Oct. 1, 2010, andclaims the benefit of priority thereof, and further claims the benefitunder 35 U.S.C. § 120 of U.S. patent application Ser. No. 12/146,976,filed Jun. 26, 2008, which in turn claims the benefit of 35 U.S.C. §119(e) to U.S. provisional application Ser. No. 60/929,397 filed Jun.26, 2007, and U.S. provisional application Ser. No. 61/026,890 filedFeb. 7, 2008, all of whose disclosures are hereby incorporated byreference in their entireties into the present disclosure.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under grant numbersEY015836 and EY001319 awarded by the National Institutes of Health. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a method of using a laser to modify therefractive index of ocular tissues, e.g., the corneal stroma or lenscortex, for vision correction.

BACKGROUND OF THE INVENTION

Conventional ultraviolet nanosecond excimer lasers have been verysuccessfully used for corneal refractive surgery such as photorefractivekeratectomy (PRK), laser-assisted in situ keratomileusis (LASIK) andlaser sub-epithelial keratomileusis (LASEK). By ablating corneal tissuethrough direct, one-photon absorption of ultraviolet light, these lasersare able to alter the curvature and thickness of corneas, ultimatelyaltering their optical power.

The rapid development of femtosecond laser technology has provided anadditional tool for corneal refractive surgery. In contrast to thephoto-ablative ultraviolet lasers, femtosecond laser pulses in the nearinfrared or visible range can pass through transparent corneal tissuewithout significant one-photon absorption. Only when pulses are focusedinside the cornea, is the intensity of the beam sufficient to causenonlinear, typically, multi-photon absorption. Because the absorption isnonlinear, the laser-affected region tends to be highly localized,leaving the surrounding region unaffected, or minimally affected. See,Vogel A, Noack J, Huttman G, Paltauf G, Mechanisms of femtosecond lasernanosurgery of cells and tissues. Applied Physics B 2005, 81, 1015-47;Loesel F H, Niemz M H, Bille J F, Juhasz T, Laser-induced opticalbreakdown on hard and soft tissue and its dependence on the pulseduration: experiment and model. IEEE Journal of Quantum Electronics1996, 32, 1717-22; and Giguere D, Olivie G, Vidal F, et al., Laserablation threshold dependence on pulse duration for fused silica andcorneal tissues: experiments and modeling, Journal of the OpticalSociety of America A 2007, 24, 1562-68. Also, several studies on theeffects of high-repetition-rate femtosecond lasers on fused silica andborosilicate glass have found that laser pulses greatly increased thetemperature of the materials at the laser focus. See, Eaton et al,Optics Express 2005, 13, 4708-16. Vogel calculated the temperaturechange in water would be >10° K with a 0.6 NA focusing lens and 100 fslaser pulses assuming that with each pulse, an energy density of 1 J/cm³at the center of the initial temperature distribution is deposited.

In the past two decades, extensive experimental and theoretical work hasbeen done to characterize laser-induced optical breakdown thresholds indifferent materials, including the cornea and the lens. Most of thiswork, however, centered on the use of continuous wave (CW) lasers or onsingle pulses from low repetition rate lasers in which thermal diffusiontime is much shorter than the time interval between adjacent pulses.Thus, each pulse is responsible for a change in the material. Indeed, ithas been established that for pulses longer than 10 ps, the opticalbreakdown threshold fluence scales as the square root of the pulseduration. To date, most femtosecond lasers used to cut corneas inclinical practice use microJoule (μJ) femtosecond laser pulses with alow-repetition-rate (Hz-kHz range) and spot diameters of more than 5microns (μm). See, Kurtz R M, Horvath C, Liu H H, Krueger R R, Juhasz T,Lamellar refractive surgery with scanned intrastromal picosecond andfemtosecond laser pulses in animal eyes, Journal of Refractive Surgery1998, 14, 541-48; and Juhasz T, Loesel C, Horvath C, Kurtz R M, MourouG, Corneal refractive surgery with femtosecond lasers, IEEE Journal ofQuantum Electronics 1999, 5, 902-09.

This contrasts with the range of femtosecond laser parameters that havebeen established for biomedical applications. See, Loesel F H, Niemz MH, Bille J F, Juhasz T, Laser-induced optical breakdown on hard and softtissue and its dependence on the pulse duration: experiment and model,IEEE Journal of Quantum Electronics 1996, 32, 1717-22. Compared with thelow-repetition-rate femtosecond lasers with μJ or milliJoule (mJ) pulseenergies, high-repetition-rate (>1 MHz) femtosecond laser oscillatorsusually have pulse energies on the order of nanoJoule (nJ). Suchlow-pulse-energy femtosecond lasers have been used for bothmicromachining and nanosurgery. See, König K, Krauss O, Riemann I,Intratissue surgery with 80 MHz nanojoule femtosecond laser pulses inthe near infrared, Optics Express 2002, 10, 171-76.

U.S. Patent No. to Hansel generically describes a method and a devicefor irradiation of ocular tissues that can be used for such applicationsas refractive surgery and laser medicine. The method described is saidto combine the “working principles of specific optical and electronicmodules to expose the eye lens to controlled therapeutic radiation inthe long-wave UV-A range above cornea absorption and/or the visibleand/or the near infra-red ranges and/or the cornea in a defined way totreatment radiation in the near infra-red wavelength range about 1.3micrometers” (see, Hansel Abstract). The therapeutic radiation is alsosaid to provide “locally photo-induced irreversible chemical changes arecreated in the eye lens substance and/or the cornea substance such thatthe refractive index and/or the transmission properties for visibleuseful radiation can be changed to pre-defined parameters, resulting ina defect-reduced vision.” Id.

While most femtosecond laser surgical procedures involve (by definition)some sort of disruption, either affecting membranes, organelles or othercellular components, they can be performed with such precision andselectivity so as not to kill the cells. Recently, research within ourgroup on both silicone and non-silicone-based hydrogels, demonstratesthat femtosecond micromachining works by inducing a significant changein refractive index of the materials without visible plasma luminescenceor bubble formation, and without the generation of undesirablescattering or absorbing centers. See, U.S. patent application Ser. No.11/745,746, filed May 8, 2007, and Ser. No. 11/948,298 filed Nov. 30,2007. Our success with creating refractive structures in hydrogelmaterials led us to explore whether similar type structures could becreated in ocular tissues.

There exists an ongoing need for ways to improve or correct vision.Changing the refractive index of ocular tissues, e.g., the cornealstroma or lens cortex, using a femtosecond laser, without tissuedestruction or wound healing response would represent a major advance inthe field of laser refractive correction or vision correction generally.

SUMMARY

An embodiment of the invention is directed to a method for forming arefractive structure in a living eye. The method includes the steps ofdirecting and focusing a plurality of femtosecond laser pulses in aspectral region between about 350 nanometers (nm) to about 600 nm withina cornea or a lens of the living eye; controlling the intensity of thelaser pulses to have an intensity sufficient to change the refractiveindex of the cornea or lens within a defined focal region, but below adamage threshold the cornea or lens, or at a level that will notphoto-disrupt cornea or lens tissue outside of the focal region; andforming a refractive structure in the focal region of the cornea or thelens by scanning the laser pulses through a volume of the cornea or thelens. Each refractive structure is characterized by a change inrefractive index, and exhibits little or no scattering loss.

An embodiment of the invention is directed to a method for providingvision correction to a patient. The method includes: (a) measuring thedegree of vision correction needed by the patient and determining thelocation and shape of a refractive structure that needs to be positionedwithin the cornea to partially correct a patient's vision; (b) directingand focusing femtosecond laser pulses in the blue spectral region withinthe cornea at an intensity high enough to change the refractive index ofthe cornea within a focal region, but not high enough to damage thecornea or to affect cornea tissue outside of the focal region; and (c)scanning the laser pulses across a volume of the cornea or the lens toprovide the focal region with a refractive structure in the cornea orthe lens. Again, the refractive structure is characterized by a changein refractive index, and exhibits little or no scattering loss.

An embodiment of the invention is directed to a method for forming arefractive structure in a living eye, comprising: directing and focusinga plurality of femtosecond laser pulses in a spectral region betweenabout 350 nanometers (nm) to about 600 nm within a defined focal regionin the cornea or lens of the living eye, wherein the laser pulses have arepetition rate from 10 MHz to 300 MHz, a pulse duration of 30 fs to 200fs, an average power from 20 mW to 160 mW, and a pulse energy from 0.01nJ to 10 nJ; further wherein the defined focal region is in the form ofa cylindrical volume having a diameter between about 1.0 μm to 2 μm anda length between about 3 μm to 6 μm; and forming a refractive structurein the focal region of the cornea or the lens, further comprisingcreating a difference in the refractive index of the refractivestructure from that outside of the focal region by between about 0.005to 0.06 without photo-disrupting cornea or lens tissue outside of thefocal region. According to an aspect, the spectral region is betweenabout 375 nm to about 425 nm. According to an aspect, the spectralregion is between about 350 nm to about 400 nm. According to an aspect,the laser pulses have a wavelength of about 400 nm. According to anaspect, the pulse energy is between about 0.1 nJ to 2 nJ. According toan aspect, the method further comprises forming the refractive structurehaving a structural form of at least one of a lens, a prism, a Bragggrating, a microlens arrays, a zone plate, a Fresnel lenses, and acombination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodied invention will be better understood from the followingdescription and in consideration with the accompanying figures. It is tobe expressly understood, however, that each of the figures is providedto further illustrate and describe the invention and is not intended tofurther limit the invention claimed.

FIG. 1 is a schematic representation of a three-dimensional structure inthe cornea stroma that can be produced by the method described;

FIG. 2 is a schematic representation of creating a convex, plano orconcave structure in cornea stroma to yield a positive or negativevision correction by the method described;

FIG. 3 is a schematic representation of the laser and optical systemused to provide the refractive structures;

FIGS. 4A and 4C are Differential Interference Contrast (DIC)photographic images of a line grating in lightly-fixed cat cornealstroma at or near the tissue breakdown threshold;

FIGS. 4B and 4D are Bright Field (BF) photographic images of a linegrating in lightly-fixed cat corneal stroma at or near the tissuebreakdown threshold;

FIG. 5A is a DIC photographic image of a line grating in lightly-fixedcat lens cortex at or near the tissue breakdown threshold;

FIG. 5B is a BF photographic image of a line grating in lightly-fixedcat lens cortex at or near the tissue breakdown threshold;

FIG. 6A is a DIC photographic image of a line grating in lightly-fixedcat corneal stroma below the tissue breakdown threshold;

FIG. 6B is a zoomed-in DIC image of the line grating refractivestructure shown in FIG. 6A;

FIG. 6C is a BF photographic image of a line grating in lightly-fixedcat corneal stroma below the tissue breakdown threshold;

FIG. 7A is a DIC photographic image of a line grating in lightly-fixedcat lens cortex below the tissue breakdown threshold;

FIG. 7B is a BF photographic image of a line grating in lightly-fixedcat lens cortex below the tissue breakdown threshold;

FIG. 8A is a graph plotting the 2nd and 3rd order diffractionefficiencies and the corresponding laser-induced refractive indexchanges of eight gratings micromachined in different corneal samples;

FIG. 8B is a photograph of the diffraction pattern obtained with a 632.8nm He—Ne laser when illuminating the grating of FIG. 6A

FIG. 9A is a graph plotting the 2nd and 3rd order diffractionefficiencies of eight gratings micromachined in different cornealsamples after one month of storage;

FIG. 9B is a photograph of the diffraction pattern obtained with a 632.8nm He—Ne laser when illuminating the grating of FIG. 6A after one month;

FIG. 10A is a graph plotting the 1st and 2nd order diffractionefficiencies and the corresponding laser-induced refractive indexchanges of eight gratings micromachined in different lens cortexsamples;

FIG. 10B is a photograph of the diffraction pattern obtained with a632.8 nm He—Ne laser when illuminating the grating of FIG. 7A;

FIG. 11A is a DIC photograph showing the line grating of FIG. 7A afterone month of storage;

FIG. 11B is a graph plotting the 1st and 2nd order diffractionefficiencies of eight gratings micromachined in different lens cortexsamples after one month of storage;

FIG. 11C is a photograph of the diffraction pattern obtained with a632.8 nm He—Ne laser when illuminating the grating of FIG. 7A after onemonth;

FIG. 12 is a schematic representation of a use of the preferred oranother embodiment in providing fiducial marks in the cornea;

FIGS. 13A-13C are schematic diagrams of a device in which the preferredor another embodiment can be implemented; and

FIG. 14 is a phase contrast photographic image of a line grating in catcorneal stroma below the tissue breakdown threshold (middle linepattern) bordered by damage lines using 800 nm femtosecond laser pulses.

DETAILED DESCRIPTION OF EMBODIMENTS THE INVENTION

Using very high-repetition-rate, ultra-short laser pulses we determinedthat the optical breakdown threshold for a 0.70 NA focusing condition inlightly-fixed cornea stroma and lens cortex is from about 40 mW to 90 mWaverage laser power, respectively. For both cornea stroma and lenscortex both values are lower than the optical breakdown power reportedby König and colleagues using 1 nJ pulse energy, 170 fs pulse durationand 1.30 NA focusing in porcine corneas. See, König et al, OpticsExpress 2002, 10(3), 171-76. By using 30 mW and 45 mW average laserpower (0.3 nJ and 0.5 nJ pulses), we discovered that one can induceIntra-tissue Refractive Index Shaping (IRIS), without accompanyingphoto-disruption and tissue destruction.

We adapted our femtosecond micromachining approach with hydrogelmaterials to carry out IRIS in biological tissues. We initially measuredthe optical breakdown thresholds of lightly-fixed cat corneas andlenses. We then reduced femtosecond laser pulse energies below theseoptical breakdown thresholds to create grating patterns that areassociated with a change in the refractive index of the tissue. Ourinvestigation has led to the development of a process to modify therefractive index of ocular tissue, e.g., corneal stroma and lens cortex,without apparent tissue destruction. Accordingly, a determination of theappropriate laser parameters is important for achieving IRIS inbiological tissues. Not only does the femtosecond laser fluence at theobjective focus have to be below the optical breakdown threshold of thetissue, the laser fluence must be strong enough to induce nonlinearchanges in the tissues. Moreover, the scan speed must be set within aspecified range.

The process involves irradiating the ocular tissue with a highrepetition, low-pulse-energy, femtosecond laser. If very short laserpulses having a very select energy are focused on ocular tissue, thetotal intensity of light leads to a change in the refractive index ofthe ocular tissue in the focal region. Moreover, the region of theocular tissue just outside the focal region is minimally affected by thelaser light. As a result, select volumes of ocular tissue can bemodified resulting in a change in the refractive index in these tissuevolumes. Moreover, the long-term stability of the observed change inrefractive index suggests permanent molecular and/or structural changesto the ocular tissue

An embodiment of the invention is directed to a method for formingrefractive structures in a living eye. The method includes (a) directingand focusing femtosecond laser pulses in the blue spectral region withina cornea or a lens of the living eye at an intensity high enough tochange the refractive index of the cornea or lens within a focal region,but not high enough to damage the cornea or lens or to affect cornea orlens tissue outside of the focal region; and (b) scanning the laserpulses across a volume of the cornea or the lens to provide the focalregion with refractive structures in the cornea or the lens. Therefractive structures exhibit little or no scattering loss, which meansthat the structures are not clearly visible under appropriatemagnification without contrast enhancement.

In one embodiment, the method can further include measuring the degreeof vision correction needed by a patient following cataract surgeryprior to step (a), and determining the location and shape of therefractive structures that need to be positioned within the cornea tocorrect the patient's vision. In another embodiment, the method canfurther include measuring the degree of vision correction needed by apatient prior to step (a), and determining the location and shape of therefractive structures that need to be positioned within the cornea tocorrect the patient's vision.

In another embodiment, the determined change in refractive index inducedin cornea and lens tissue using the described process is relativelysmall, but very significant. Based on published values for the power(39D) and native refractive index (1.376) of the cat cornea, IRIS shouldgenerate a change in corneal power ranging between 0.14D and 0.56D(assuming an index change between 0.005 and 0.02). Similarly, for thecat lens (power=53D, refractive index of the homogeneous lens=1.554),the refractive index changes induced by micromachining shouldtheoretically alter lenticular power by between 0.5D and 0.7D. The laserprocess described could completely alter the approach to laserrefractive surgery and to vision correction.

In addition, the preservation of tissue clarity during the treatmentallows the application of IRIS for the creation of corneal fiducialmarkings that could be used to align eye trackers during LASIK, and forrefractive corrections in a closed-loop approach, e.g. with specificbenefit for the correction of higher-order aberrations, as well as for“touch-up corrections” of ocular surface defects. Various types ofrefractive structures can be created in biological tissues. Examplesinclude high refractive index structures such as Bragg gratings,microlens arrays, optical zone plates, and Fresnel lenses.

As stated, the determination and selection of the laser operatingparameters are particularly important in implementing IRIS. Theinventors have found that various ranges of parameters are particularlyuseful in implementing the present invention. In treatment of the eye,the laser wavelength should be such that the tissues through which laserpulses pass are transparent to the pulses. There should also be nodamage to the retina; any change should be confined to the tissue withinthe focal region. Also, for non-destructive alteration of ocular tissue,a CO₂ laser or excimer laser should not be used, since there should beno ablation or removal of the tissue.

A laser pulse frequency from 1 MHz to 10 GHz, and preferably from 10 to300 MHz, should be used. For example, our work used a laser pulsefrequency (repetition rate) of 70 MHz to 100 MHz, e.g., about 93 MHz orabout 80 MHz.

Linked to the pulse frequency is a pulse duration of about 30 fs toabout 200 fs. For example, a laser pulse duration of 80 fs to 120 fs.

Linked to the pulse frequency is the average laser power. A preferableaverage laser power is from 1 mW to 1,000 mW, e.g., from 20 mW to 160mW, and more preferably from 60 mW to 110 mW.

The energy of each pulse should be in a range from 0.01 nJ to 10 nJ,preferably from 0.1 nJ to 2 nJ, and more preferably less than 1 nJ. Forexample, we have determined that a pulse energy from 0.1 nJ to 0.5 nJ,is a preferred energy range.

The laser pulse will have a peak intensity at focus of greater than 10¹³W/cm². At times, it may be advantageous to provide a laser with a peakintensity at focus of greater than 10¹⁴ W/cm², or greater than 10¹⁵W/cm².

We found that by using femtosecond laser pulses in the blue spectralregion one can achieve high native nonlinear absorption so that theobserved changes in index of refraction are strongly localized in threedimensions. The wavelength is chosen such that there is minimal visualsensitivity, high retinal damage threshold, and no UV photophysicalone-photon damage mechanisms induced in the eye. And furthermore, thetechnique is performed in such a manner that there is minimal death tothe live cells in the stroma, which can reduce the wound healingresponse from the procedure.

Linked to one or more of the above laser pulse parameters is the speed(mm/s) at which the laser pulses is scanned across a volume of theocular tissue. Although scanning speeds as low as 0.05 mm/s can be useddepending on the equipment, types of structures to be written and typeof ocular tissue, greater scan speeds in a range from 0.1 mm/s to 30mm/s are generally preferred. We have generally utilized scan speeds of1 mm/s to 15 mm/s, e.g., 1 mm/s, 5 mm/s, 10 mm/s and 15 mm/s keeping allother laser parameters constant (wavelength 400 nm; average laser power80 mW, pulse duration 100 fs to form refractive structures 150 μm intothe corneal stroma.

The refractive structures are formed by scanning the laser pulses acrossa volume of ocular tissue. In theory, each short series of laser pulsesis believed to form cylindrical volumes from about 0.5 μm to 3 μm indiameter and 3 μm to 10 μm in length. By scanning the laser pulsesacross the tissue the cylindrical volumes form continuous refractivestructures in two or three dimensions. In one embodiment, the focalregion can be defined by a cylindrical volume from about 1.0 μm to 2 μmin diameter and 3 μm to 6 μm in length.

The pulse energy of the focused laser used in the method will in-partdepend on the type of structures to be written into the ocular tissue,the type of ocular tissue and how much of a change in refractive indexis desired. The selected pulse energy will also depend upon the scanrate at which the structures are written into the ocular tissue.Typically, greater pulse energies will be needed for greater scan rates.

The pulse width must be preserved so that the pulse peak power is strongenough to exceed the nonlinear absorption threshold of the oculartissue. However, the glass of the focusing objective(s) significantlyincreases the pulse width due to the positive dispersion of the glass. Acompensation scheme is used to provide a corresponding negativedispersion that can compensate for the positive dispersion introduced bythe focusing objective(s). Accordingly, the term “focused” in thisapplication refers to the focusing of light from a laser within oculartissue using a compensation scheme to correct for the positivedispersion introduced by the focusing objective(s). The compensationscheme can include an optical arrangement selected from the groupconsisting of at least two prisms and at least one mirror, at least twodiffraction gratings, a chirped mirror, and dispersion compensatingmirrors to compensate for the positive dispersion introduced by thefocus objective.

In one embodiment, the compensation scheme comprises at least one prism,in many cases at least two prisms, and at least one mirror to compensatefor the positive dispersion of the focusing objective. In anotherembodiment, the compensation scheme comprises at least two gratings tocompensate for the positive dispersion of the focusing objective. Anycombination of prisms, gratings and/or mirrors can be used for thecompensation scheme in accordance with optical principles known by thoseof ordinary skill in the art.

As stated, the refractive structures can be defined by two- orthree-dimensional structures. The two- or three-dimensional structurescan comprise an array of discrete cylinders. Alternatively, the two- orthree-dimensional structures can comprise a series of lines (a grating)or a combination of an array of cylinders and a series of lines.Moreover, the two- or three-dimensional structures can comprise area orvolume filled structures, respectively. These area or volume filledstructures are formed by continuously scanning the laser over a selectplane or volume of the ocular tissue, respectively. As stated, varioustypes of refractive structures can be created in biological tissues.Examples include high refractive index structures such as lenses,prisms, Bragg gratings, microlens arrays, optical zone plates, andFresnel lenses.

The area-filled or volume-filled two- or three-dimensional structurescan be formed by continuously scanning the laser over select volumes ofthe ocular tissue. Refractive-type optical devices can be micro-machinedinside the volume of ocular tissue by repeatedly scanning a tightlyfocused beam of femtosecond pulses in an area segment. The area of thesegment can be changed correspondingly with the depth of the scan, so asto produce three-dimensionally shaped lenses with spheric, aspheric,toroidal or cylindrical shapes as shown in FIG. 1. Alternatively,refractive corrective lenses can be made in various combinations ofconvex, plano- or concave to yield a positive correction, or negativecorrection, as shown in FIG. 2. The refractive optical devices can bestacked vertically, written separately in different planes, so as to actas a single lens. Additional corrective layers can be written asdesired.

In one embodiment, the focal region of the ocular tissue is defined by aseries of lines in an approximately two dimensional plane having a widthfrom 0.2 μm to 3 μm, preferably a width from 0.6 μm to 1.5 μm and aheight from 0.4 μm to 8 μm, preferably a height from 1.0 μm to 4 μm(height is measured in the z direction, which is parallel to directionof the laser light). For example, one can generate a line gratingcomprising a plurality of lines with each line of any desired length,about 0.8 μm to about 5 μm, about 0.8 μm to about 3 μm or about 0.8 μmto about 1.5 μm in width and about 2 μm to about 10 μm about 2 μm to 5μm in height. The lines can be separated by as little as 1.0 μm (0.5 μmspacing), and any number of lines can be incorporated into the oculartissue. Moreover, the grating can be positioned at any selected depth(z-direction), and any number of line gratings can be generated atvarious depths into the ocular tissue.

In one embodiment, the refractive structures placed in corneal stromawill exhibit a change in the index of refraction of about 0.005 to about0.06, and typically about 0.01 to 0.04. This observed change in theindex of refraction is relative to the bulk cornea stroma outside thefocal region. Based on published values for the power (39D) and nativerefractive index (1.376) of the cat cornea, the refractive index changesinduced by micromachining should generate a change in corneal powerranging between 0.1D and 01.0D or 0.1D and 0.5D (assuming thatrefractive index change affects the thickness of the cornea uniformly).

Our initial work with 800 nm light demonstrated that it is possible tocause low-scattering-loss, refractive index modifications inlightly-fixed cat cornea and lens using 93 MHz repetition rate, 27 fslaser pulses with 0.3 nJ and 0.5 nJ pulse energies. The use ofnear-infrared light that is just beyond the visual response on the longwavelength end is desirable for use in live eyes, since it would provideminimal retinal stimulation and eye aversion response. Thesemodifications were visible only using DIC microscopy and were notassociated with apparent tissue damage. The resulting modificationscorrespond to refractive index changes between 0.05±0.001 and0.021±0.001. Preservation of IRIS over a month of refrigerated storagesuggested that the femtosecond laser-induced modifications were likelyto involve relatively long-term molecular/structural alterations. Inrelated experiments involving silicone hydrogels, the micromachinedgratings (and associated Raman spectra) are stable for at least oneyear, even after drying and rehydration of the hydrogel.

For example, it is possible to cause low-scattering-loss, refractiveindex modifications in lightly-fixed cat cornea and lens using 93 MHzrepetition rate, 27 fs laser pulses with 0.3 nJ and 0.5 nJ pulseenergies. These modifications were visible only using DIC microscopy andwere not associated with apparent tissue damage. The resultingmodifications correspond to refractive index changes between 0.05±0.001and 0.021±0.001. Preservation of IRIS over a month of refrigeratedstorage suggested that the femtosecond laser-induced modifications werelikely to involve relatively long-term molecular/structural alterations.In related experiments involving silicone hydrogels, the micromachinedgratings (and associated Raman spectra) are stable for at least oneyear, even after drying and rehydration of the hydrogel.

In the spectral region around 400 nm, the human cornea starts absorbingslightly. At shorter wavelengths, this absorption becomes moresignificant. In a spectral window in the blue, the spectral response isminimal, yet the linear absorption is not is low enough not to causephoto-disruption of the tissue. Linear absorption in the cornea wouldcause unwanted attenuation of the excitation beam and unlocalizedheating of the corneal tissue, which would result in cell death, whichis undesirable. While the spectral region near 400 nm satisfies bothrequirements of minimized visual response as well as minimized linearabsorption, forming of the refractive structures according to theembodied invention may be carried out between about 350 nm to about 600nm within the defined focal region. An advantageous spectral range maybe between about 375 nm to about 425 nm. Another advantageous spectralrange may be between about 350 nm to about 400 nm.

A Laser and Optical Configuration for Modifying Ocular Tissue

A non-limiting embodiment of a laser system 10 for irradiating oculartissue with a laser to modify the refractive index of the tissue inselect regions is represented in FIG. 3. A laser source comprises aKerr-lens mode-locked Ti:Sapphire laser 12 (Kapteyn-Murnane Labs,Boulder, Colo.) pumped by 4 W of green light from a frequency-doubledNd:YVO₄ laser 14. The laser generates pulses of 300 mW average power, 30fs pulse width and 93 MHz repetition rate at wavelength of 800 nm.Because there is a reflective power loss from the mirrors and prisms inthe optical path, and in particular, from the power loss of theobjective 20, the measured average laser power at the objective focus onthe material is about 120 mW, which indicates the pulse energy for thefemtosecond laser is about 1.3 nJ.

The same laser source that is used to generate 800 nm femtosecond laserpulses directly above can also be used to generate a 400 nm femtosecond(fs) laser pulse using laser optical methods and devices well known inthe art. For example, we have used the Kerr-lens mode-locked Ti:Sapphirelaser to generate 400 nm fs laser pulses with an average laser power ofabout 80 mW, and a pulse duration of about 100 fs to form refractivestructures within ocular tissues, e.g., corneal stroma. As stated, theuse of the shorter wavelength laser pulse verses, for example, at 800nm, allows one to create refractive structures at much greater scanspeeds for a given change in refractive index of the tissue. Also, veryimportantly, the shorter wavelength laser pulse allows one to make muchlarger changes in the refractive index of the ocular tissue that was notpossible at 800 nm without approaching the damage threshold of thetissue.

Due to the limited laser pulse energy at the objective focus, the pulsewidth must be preserved so that the pulse peak power is strong enough toexceed the nonlinear absorption threshold of the ocular tissue. Becausethe glass inside the focusing objective significantly increases thepulse width due to the positive dispersion inside of the glass, anextra-cavity, compensation scheme is used to provide the negativedispersion that compensates for the positive dispersion introduced bythe focusing objective. Two SF10 prisms 24 and 28 and one ending mirror32 form a two-pass one-prism-pair configuration. We used a 37.5 cmseparation distance between the prisms to compensate the dispersion ofthe microscope objective and other optics within the optical path. Acollinear autocorrelator 40 using third-order harmonic generation isused to measure the pulse width at the objective focus. Both 2nd and 3rdharmonic generation have been used in autocorrelation measurements forlow NA or high NA objectives. We selected third order surface harmonicgeneration (THG) autocorrelation to characterize the pulse width at thefocus of the high-numerical-aperture objectives because of itssimplicity, high signal to noise ratio and lack of material dispersionthat second harmonic generation (SHG) crystals usually introduce. TheTHG signal is generated at the interface of air and an ordinary coverslip 42 (Corning No. 0211 Zinc Titania glass), and measured with aphotomultiplier 44 and a lock-in amplifier 46. After using a set ofdifferent high-numerical-aperture objectives and carefully adjusting theseparation distance between the two prisms and the amount of glassinserted, we selected a transform-limited 27-fs duration pulse, which isfocused by a 60×0.70 NA Olympus LUCPlanFLN long-working-distanceobjective 48.

Because the laser beam will spatially diverge after it comes out of thelaser cavity, a concave mirror pair 50 and 52 is added into the opticalpath in order to adjust the dimension of the laser beam so that thelaser beam can optimally fills the objective aperture. A 3D 100 nmresolution DC servo motor stage 54 (Newport VP-25XA linear stage) and a2D 0.7 nm resolution piezo nanopositioning stage (PI P-622.2CD piezostage) are controlled and programmed by a computer 56 as a scanningplatform to support and locate the samples. The servo stages have a DCservo-motor so they can move smoothly between adjacent steps. An opticalshutter controlled by the computer with 1 ms time resolution isinstalled in the system to precisely control the laser exposure time.With customized computer programs, the optical shutter could be operatedwith the scanning stages to micro-machine different patterns in thematerials with different scanning speed at different position and depthand different laser exposure time. In addition, a CCD camera 58 alongwith a monitor 62 is used beside the objective 20 to monitor the processin real time.

The method and optical apparatus described above can be used to modifythe refractive index of ocular tissue as follows. The first step in ourmicromachining experiment was to establish thresholds for the opticalbreakdown of lightly fixed feline cornea and lens cortex. The neutraldensity filter was first adjusted to minimize the focused incident laserpower on the cornea and the lens below their breakdown thresholds. Theincident laser power was then progressively increased by adjusting theneutral density filter. The breakdown threshold power was considered tobe reached when visible plasma luminescence suddenly appeared and strongscattering light as well as laser-induced damage became visible, seeFIGS. 4A to 4D and FIGS. 5A and 5B. Using the 0.70 NAlong-working-distance objective in our system, the measured breakdownthresholds for cat cornea and lens was about 55 mW and 75 mW averagelaser power, respectively, which corresponds to a pulse energy of 0.6 nJand 0.8 nJ, respectively.

FIGS. 4A to 4D are microscopic photographs of line gratingsmicromachined in lightly-fixed, cat corneal stroma using femtosecondlaser conditions at or near the tissue breakdown threshold. FIGS. 4A and4C are Differential Interference Contrast (DIC) images of lines createdin the stroma of two different, lightly-fixed cat corneas with 0.6 nJpulses and a scanning speed of 10 μm/s. Note, the spots of tissuedestruction or “bubbles” (arrowed) along the micromachined lines (theclear, horizontal lines within stroma tissue). FIGS. 4B and 4D areBright Field (BF) images of the same line gratings of FIGS. 4A and 4C,respectively. The BF images illustrate the visibility of tissuedestruction (arrowed) and the relative invisibility of the rest of thelines that are clearly seen under DIC conditions.

Once tissue breakdown thresholds were established, the focused laserpower was lowered gradually by carefully adjusting the neutral densityfilter until lines could be micromachined without the induction ofbubbles or burns. We determined an average laser power setting of 30 mWfor the cornea, which corresponds to a pulse energy of about 0.3 nJ.

The gratings were micromachined in the horizontal plane within thestroma of each corneal piece at a constant speed of 0.7 μm/s. Thegratings consisted of 20-40 parallel lines, 100 μm long, 1 μm linewidth,5 μm apart and about 100 μm beneath the corneal epithelium. Likewise,gratings were micromachined in the horizontal plane within the cortex ofeach lens at a constant speed of 1.0 μm/s. The gratings again consistedof 20-40 parallel lines, 100 μm long, 1 μm linewidth, 5 μm apart andabout 100 μm beneath the lenticular surface. The spherical aberration atthe laser focus induced by refractive index mismatch was compensated byan adjustable cover slip correction of the focusing microscope objectivein order to achieve the smallest possible laser-affected region alongthe laser propagation direction.

Observation and Measurement of Refractive Index Change.

After writing the observed structures in both corneal stroma and lenscortex we assessed whether the micromachined gratings are associatedwith a change in refractive index of the two different tissues.Immediately after micromachining, the slide containing the corneal pieceand lens cortex was examined under an Olympus BX51 optical microscope.Bright field, phase contrast (PC) and differential interference contrast(DIC) were used to view the gratings. The slide was then moved toanother setup where a low power 632.8 nm He—Ne laser was used toirradiate the gratings. The diffraction pattern from each grating wascaptured by a digital camera. The refractive index changes attained werecalculated as described previously. See, Ding L, Blackwell R, Künzler JF, Knox W H, Large refractive index change in silicone-based andnon-silicone-based hydrogel polymers induced by femtosecond lasermicro-machining, Optics Express 2006, 14, 11901-909.

In brief, the intensity of 0th order to 3rd order of diffracted lightfrom the gratings was measured by a power meter. The different orderdiffraction efficiencies was obtained by calculating the ratios betweenthe intensity of 1st, 2nd and 3rd to 0th order diffraction light.Because only one particular value of the refractive index change matchesone particular diffraction efficiency value, one could calculate theindex change within the femtosecond laser micromachined regions. We notethat several factors could affect the results, such as the accuracy ofmeasurement for the different diffraction order intensities, and themeasurements of grating linewidth and thickness. To reduce measurementerror of the diffraction order intensities, we took five measurements oneach grating and calculated the average value and the standard deviationof the results. In principle, the spatial distribution of the refractiveindex change within the micromachined region was a small-scalegradient-index structure. For the purpose of this investigation,however, we presumed the index profile to be uniform within the gratinglines, which were only 3 μm deep because the spherical aberration at thefocal point was corrected.

The micromachined cat cornea and lens pieces were then removed from theglass slides after discarding the cover slips, and stored in theethylene glycol/sucrose solution at 4° C. After one month, each cornealpiece and lens piece was mounted onto a new glass slide for imaging andthe diffraction light intensity measurement was repeated. This allowedus to assess whether the refractive index change initially observed hadbeen maintained during storage.

Exposure of lightly-fixed cat corneal to 0.3 nJ femtosecond laser pulses(30 mW average laser power) resulted in the reliable creation of gratingpatterns about 100 μm below the epithelial surface in all test samples,even when they were obtained from different cats. When imagedimmediately after micromachining, individual grating lines could beclearly observed and distinguished with DIC microscopy (FIGS. 6A and6B), but they were practically invisible when viewed under bright fieldtransmission microscopy (FIG. 6C). This could be interpreted as thegrating lines having very low scattering properties, which is in greatcontrast to the destructive tissue changes observed when laser energywas increased above the optical breakdown threshold levels (spots inFIG. 4). Using the knife-edge method, we ascertained that the laserfocus diameter was 2.5 μm in air, which was much bigger than themicromachined line-widths. Therefore, it appears that only the centralpart of the laser focal area had sufficient intensity to modify therefractive index of corneal tissue.

Likewise, exposure of lightly-fixed cat lens cortex to 0.5 nJfemtosecond laser pulses (45 mW average laser power) resulted in thereliable creation of grating patterns about 100 μm below the lenticularsurface in all test samples, even when they were obtained from differentcats. When imaged immediately after micromachining, individual gratinglines could be clearly observed and distinguished with DIC microscopy(FIG. 7A), but they were practically invisible when viewed under brightfield transmission microscopy (FIG. 7B). Again, this is interpreted asthe grating lines having very low scattering properties, which is ingreat contrast to the destructive tissue changes observed when laserenergy was increased above the optical breakdown threshold levels (spotsin FIG. 5). Also, it appears that only the central part of the laserfocal area had sufficient intensity to modify the refractive index oflens cortex.

In order to further assess the optical consequences of low-pulse-energyfemtosecond laser micromachining on corneal stroma and lens cortex, weimmediately irradiated the micromachined gratings with low power 632.8nm He—Ne laser light. Because displacement of the stroma collagenlamellae as a result of post-mortem corneal swelling could not becompletely avoided, scattering effect from the 0th order diffractionlight was very strong, obscuring the 1st order diffraction light. Thus,only the 2nd and 3rd order diffraction efficiencies of each gratingcould be measured and used to calculate an approximate refractive indexchange within the femtosecond laser micromachined regions, FIG. 8A. Incontrast, tissue swelling and opacification were minimal in slices oflens cortex, the 0th through 3rd order diffraction light could bemeasured clearly, and 1st and 2nd order diffraction efficiencies wereused to calculate the induced change in refractive index (FIG. 10A).

Although a single diffraction efficiency is usually sufficient tocalculate refractive index, we measured 1st/2nd or 2nd/3rd combinationsto confirm that the refractive indices calculated were consistentthrough different diffraction orders. For these calculations, theaverage refractive indices of cat corneal stroma and lens were assumedto be 1.376 and 1.400, respectively. For corneal stroma, the calculatedrange of refractive index changes induced by the laser micromachiningwas from 0.005±0.001 to 0.01±0.001. For lens cortex, the calculatedrange of refractive index changes induced by the laser micromachiningwas from 0.005±0.001 to 0.03±0.001.

After undergoing low-pulse-energy femtosecond laser micromachining, eachcornea piece was returned to the storage solution in a −20° C. freezerfor one month in order to determine if the micromachined structurescould be maintained over such a period of time. After one month, thecornea pieces were removed from storage and re-examined. The storagesolution significantly slowed corneal swelling and opacification(relative to conventional storage in 0.1 M PBS, for example), but wasnot able to completely prevent these events. In spite of a moderate lossof corneal transparency, DIC microscopy did reveal that the gratingstructures initially micromachined into the corneal stroma were stillpresent one month after they were originally created as demonstrated bythe diffraction pattern observed in FIGS. 7B and 9B. The edges of thelens slices became opaque following one month storage, but the centersremained largely transparent and the micromachined gratings were stillclearly visible in a DIC image, FIG. 11A.

The diffraction light distribution of one-month old gratings in cornealstroma (FIG. 9B) was again measured and found to be no different thanthat obtained right after the gratings' creation (FIG. 8B). Also, thediffraction light distribution of one-month old gratings in lens cortex(FIG. 11C) was again measured and found to be no different than thatobtained right after the gratings' creation (FIG. 10B). In the cornealpieces, the scattering light from the 0th order diffraction stillobscured the 1st order diffraction. However, the 2nd, 3rd, and even 4thorder diffractions were still visible and easy to measure. In the caseof the 800 nm work, the measured refractive index change after one monthof storage remained from 0.005±0.001 to 0.01±0.001 for the cornealpieces and from 0.005±0.001 to 0.03±0.001 for the lens pieces.

Applications in ophthalmic surgery will now be described. As shown inFIGS. 4A to 4C, it is possible to write micron-scale features into thecorneal stroma with minimal scattering loss by carefully controlling thelaser and scan parameters such as pulse width, average power, repetitionrate, scan rate and focusing conditions. This result, which issignificantly different than the results in corneal surgery that havebeen previously reported using femtosecond, focused pulses, suggests tous certain applications.

One such application is in writing fiducial marks in the corneal stroma.More particularly, in one application involving excimer laser ablationof the cornea for vision correction—laser in situ keratomileusis orLASIK—it is first necessary to cut across the cornea with a ‘flapcutting’ device. Typically, a rapidly vibrating razor blade ormicrokeratome is used for this purpose. This method generally producesacceptable results, however the depth of the final cut is not precise,and sometimes the degree of accommodation that can be achieved withexcimer laser ablation is compromised. A competing form of cornealflap-cutting involves the use of a high-power, femtosecond laser.Femtosecond flap cutting has not been widely adopted yet in clinicalrefractive surgery practices, in part because of uncertainty about thelong-term photochemical, mechanical and biological effects of thistechnique (Stonecipher et al., 2006; Wilson et al., 2007). Recently,there have been reports about negative effects of this technique,particularly in terms of tissue destruction, which appears significantlystronger than that obtained following microkeratome cutting (Stonecipheret al., 2006; Wilson et al., 2007).

The micromachining process described provides a possible solution to theproblem of being able to make a precise cut in the corneal stromawithout additional tissue destruction. FIG. 12 shows a situation inwhich fiducial marks 101 has been machined into the stroma of thecornea, for example, at a specific location and depth. Low-energyfemtosecond laser pulses can be used to write fiducial marks 101 in thestroma of the cornea at a specific depth and location. The fiducial mark101 would not be visible to a human, as indicated by FIGS. 4b and 4D,however it is detectable by specialized optical techniques such asOptical Coherence Tomography (OCT) or Differential Interference Contrast(DIC) microscopy (FIGS. 4A and 4C).

The fiducial marks 101 could be used to ‘lock’ the depth of the cuttingblade by using an imaging technique such as Optical Coherence Tomography(OCT). OCT has been well developed for both retinal and corneal imaging.This would ensure that the resulting depth of the blade cut would besignificantly better regulated than is currently possible, even if afemtosecond laser is used to cut the corneal flap. The location anddepth of a fiducial mark 101 is obtained using optical coherencetomography (OCT) interfaced with blade control, and the depth of theblade 105 is ‘locked’ to this depth and ‘guided’ along a specifiedcutting path, which can ensure accurate flap cutting.

Another application is in altering the optical power of the cornea.Currently, laser refractive surgery achieves changes in the opticalpower of the cornea by destroying/removing corneal tissue. Tissuedestruction causes (1) a change in the surface profile (and curvature)of the cornea, (2) a change in corneal biomechanical properties (usuallya flattening of the corneal surface), and (3) a wound healing response.A change in surface shape of the cornea as a result of points (1) and(2) is sufficient to correct large optical aberrations such as defocusand astigmatism. However, as mentioned earlier, the wound healingresponse that results from corneal tissue destruction limits currentlaser refractive procedures by decreasing their ultimate opticalbenefit. Exemplary femtosecond micromachining patterns that could bewritten into the corneal stroma include a continuous circular area, anannulus pattern, or a segmented annulus pattern.

The use of femtosecond laser pulses as described to modify the opticalpower of the cornea can be accomplished as follows: (1) by changing therefractive index of the cornea stroma, and (2) by altering cornealbiomechanics without inducing a significant corneal wound healingresponse. Because of the femtosecond laser's ability to be focusednon-invasively, in a non-contact manner, to effect at any chosen depthwithin the cornea stroma, this procedure would not require removal ofthe corneal epithelium or creation of a corneal flap. Epithelialmanipulations are one of the major stimuli causing the wound healingresponse since such manipulations destroy the normally close interaction(both physically and biologically) of the corneal epithelium with itsunderlying stroma. The femtosecond micromachining (i.e. use oflow-energy femtosecond pulses to alter tissue propertiesnon-destructively) could be applied over a continuous area, 6-8 mm indiameter, in the center of the cornea or at particular locations in thecorneal periphery as mentioned above, depending on the optical orbiomechanical changes desired.

Yet another application is in altering the optical power of theintraocular lens of the eye. Presbyopia, or the loss of accommodationability as a function of age, is currently of epidemic proportions inthe developed world. The most common treatment approaches for thiscondition include reading glasses, bifocal glasses, contact lenses(including bifocals), multifocal laser refractive treatments, monovisionlaser refractive treatments and the use of accommodative intraocularlens implants into the eye. Other approaches involve the use of surgicalor destructive laser treatments to punch holes in the patient'sintraocular lens, thus decreasing its rigidity, and restoring somelimited accommodative power. However, any invasive or destructiveprocedures induce a wound healing response in the lens, with increasedrisk of opacification or cataract formation.

The micromachining process described also provides an opportunity for anocular surgeon to modify the refractive index of the corneal stromalayer of a patient having gone cataract surgery. The method allows theocular surgeon to correct any aberrations as a result of the surgery.For example, starting from a lens of selected power, the power of whichwill vary according to the ocular requirements of the patient, thesurgeon can subsequently adjust the refractive properties of the cornealstroma layer to correct a patient's vision based upon the individualneeds of the patient. In essence, an intraocular lens would essentiallyfunction as a fixed power lens to correct for the refractive error of apatient's eye. The patient's vision can then be further adjustedpost-implantation by modifying the refractive index of select regions ofthe patient's corneal stroma layer. As a result, post-operativerefractive errors resulting from pre-operative measurement errors,variable lens positioning during implantation and wound healing(aberrations) can be corrected.

For instance, cataract surgery typically requires that the natural lensof each eye be replaced with an intraocular lens (IOL). Followinginsertion of the IOL the surgeon or eye specialist can correct foraberrations resulting from the surgery or correct for slightmisplacement of the IOL. Following surgery, and after allowing time forthe wound to heal, the patient would return to the surgeon to haveselect regions of his or her corneal stroma layer irradiated. Theseirradiated regions would experience a change in refractive index, whichwould correct for the aberrations as well as the patients needs forvision correction.

Accordingly, the invention is directed to a method comprisingidentifying and measuring the aberrations resulting from the surgicalprocedure. Once the aberrations are identified and quantified usingmethods well known in the art of ophthalmology, this information isprocessed by a computer. Of course, information related to the requisitevision correction for each patient can also be identified anddetermined, and this information can also be processed by a computer.There are a number of commercially available diagnostic systems that areused to measure the aberrations. For example, common wavefront sensorsused today are based on the Schemer disk, the Shack Hartmann wavefrontsensor, the Hartmann screen, and the Fizeau and Twymann-Greeninterferometers. The Shack-Hartmann wavefront measurement system isknown in the art and is described in-part by U.S. Pat. Nos. 5,849,006;6,261,220; 6,271,914 and 6,270,221. Such systems operate by illuminatinga retina of the eye and measuring the reflected wavefront.

Once the aberrations are identified and quantified, the computerprograms determine the position and shape of the optical structures tobe written into the corneal stroma to correct for those aberrations.These computer programs are well known to those of ordinary skill in theart. The computer than communicates with the laser-optical system andselect regions of corneal stroma are irradiated with a focused, visibleor near-IR laser having a pulse energy from 0.01 nJ to 1.0 nJ.Alternatively, one can use 400 nm laser light with a similar pulseenergy to generate even greater changes in the refractive index ofcorneal stroma.

The described micromachining process can also be used for custom visioncorrection of higher order wavefront aberration in the optical path ofthe eye. The basic technology for detecting and correcting aberrationsof at least third-, fifth-, and tenth orders is taught in U.S. Pat. No.5,777,719, whose disclosure is hereby incorporated by reference in itsentirety into the present disclosure. Given that the region ofrefractive index change generated by femtosecond laser micromachiningcan be as small as 1 μm in diameter, this will make it possible tocorrect small, localized optical wavefront aberrations (higher orderaberrations) in the optical path of the eye. Such aberrations exist bothnaturally, or can be induced by ocular surgeries, such as laserrefractive surgery, corneal transplantation and wound healing followingtrauma to the eye.

FIGS. 13A to 13C show a schematic diagram of a device 1 used to carryout the preferred embodiment or another embodiment. The device 1includes a laser 3 for emitting femtosecond laser pulses, a shutter 5, afocusing lens 7, a dichroic mirror 9, a wavefront sensor 11 having alenslet array 13 and a detector array 15, and a control system 17 forcontrolling the operations described herein.

As illustrated in FIGS. 13A to 13C, the process we propose would includethe following steps: (1) using a wavefront sensor to detect and measurethe lower and higher order aberrations along the optical path of a giveneye, (2) calculating the topography and magnitude of refractive indexchanges required to achieve the necessary aberration correction, (3)focusing the femtosecond laser pulses either into the cornea orintraocular lens in order to carry out the micromachining necessary toinduce the required refractive index change. Once the micromachining iscomplete, the wavefront sensor would be used once again to check thecorrection of the ocular wavefront. Since the resolution of thefemtosecond laser micromachining is about 1 μm, this noninvasive methodcould be used as a complement or an alternative method for currentcustomized wavefront correction methods.

In FIG. 13A, the shutter 5 is closed for detection of wavefrontaberration from the optical path through the wavefront sensor 11, usingaberrated light reflected from the retina of the eye. In FIG. 13B, theshutter 5 is open, and light pulses from the femtosecond laser 3 areused to correct the aberration by locally changing the index in thecornea or the lens of the eye. In FIG. 13C, after femtosecond laser 3micromachining, the wavefront correction is verified once again usingthe wavefront sensor 11.

Calculation of Change in Refractive Index.

As mentioned, these gratings were investigated by focusing anunpolarized He—Ne laser beam with a wavelength of 632.8 nm on thesegratings and monitoring the diffraction pattern. The diffraction anglesshowed good agreement with the diffraction equationmλ=d sin θ  (1)

where m is the diffraction order, λ is the wavelength of the incidentlaser beam which here is 632.8 nm, and d is the grating period.

The diffraction efficiency of the grating can be measured, and since theefficiency is a function of the refractive index change, it may be usedto calculate the refractive index change in the laser irradiationregion. Consider the grating as a phase grating, its transmittancefunction could be written as

$\begin{matrix}{{t\left( {x_{0},y_{0}} \right)} = {{\left( {e^{i\;\phi_{2}} - e^{i\;\phi_{1}}} \right){{rect}\left( \frac{x_{0}}{a} \right)}*\frac{1}{d}{{comb}\left( \frac{x_{0}}{d} \right)}} + e^{i\;\phi_{1}}}} & (2)\end{matrix}$

where a is the grating line width, d is the groove spacing, ϕ₂ and ϕ₁are the phase delays through the lines and ambient region respectively,

${{\phi_{2}2\pi \times \frac{\left( {n + {\Delta\; n}} \right) \times b}{\lambda}\mspace{14mu}{and}\mspace{14mu}\phi_{1}} = {2\pi \times \frac{n \times b}{\lambda}}},$b is the thickness of the grating line, n is the average refractiveindex of the material, Δn is the average refractive index change in thegrating lines, and λ is the incident light wavelength of the measurement(632.8 nm). Here, the grating line width is 1 μm and the thickness is 3μm. The index change within the laser effect region can be approximatedto be uniform. The convolution theorem can be used to calculate thespectrum of the grating such asT(f _(x) ,f _(y))=F{t(x ₀ ,y ₀)}=(e ^(iϕ) ² −e ^(iϕ) ¹ )a sin c(af_(x))comb(df _(x))δ(f _(y))+e ^(iϕ) ¹ δ(f _(x) ,f _(y))  (3)

Then, the intensity distribution of the grating diffraction pattern is:

$\begin{matrix}{{I\left( {x,y} \right)} = {\left( \frac{1}{\lambda\; z} \right)^{2} \times \left\lbrack {{\left( {e^{i\;\phi_{2}} - e^{i\;\phi_{1}}} \right)\frac{a}{d}{\sum\limits_{n = {- \infty}}^{\infty}\;{\sin\;{c\left( \frac{a\; n}{d} \right)}{\delta\left( {{\frac{x}{\lambda\; z} - \frac{n}{d}},\frac{y}{\lambda\; z}} \right)}}}} + {e^{i\;\phi_{1}}{\delta\left( {\frac{x}{\lambda\; z},\frac{y}{\lambda\; z}} \right)}}} \right\rbrack^{2}}} & (4)\end{matrix}$

From this formula, the intensity of the 0th (I0), 1st (I1), and 2nd (I2)order diffraction light is

$\begin{matrix}{I_{0} = {\left( \frac{1}{\lambda\; z} \right)^{2} \times \left\lbrack {{\left( {e^{i\; 2\pi \times \frac{{({n + {\Delta\; n}})} \times b}{\lambda}} - e^{i\; 2\pi \times \frac{n \times b}{\lambda}}} \right)\frac{a}{d}} + e^{i\; 2\pi \times \frac{n \times b}{\lambda}}} \right\rbrack^{2}}} & (5) \\{{I_{1} = {\left( \frac{1}{\lambda\; z} \right)^{2} \times \left\lbrack {\left( {e^{i\; 2\pi \times \frac{{({n + {\Delta\; n}})} \times b}{\lambda}} - e^{i\; 2\pi \times \frac{n \times b}{\lambda}}} \right)\frac{a}{d}\sin\;{c\left( \frac{a}{d} \right)}} \right\rbrack^{2}}}{and}} & (6) \\{I_{2} = {\left( \frac{1}{\lambda\; z} \right)^{2} \times \left\lbrack {\left( {e^{i\; 2\pi \times \frac{{({n + {\Delta\; n}})} \times b}{\lambda}} - e^{i\; 2\pi \times \frac{n \times b}{\lambda}}} \right)\frac{a}{d}\sin\;{c\left( \frac{2\; a}{d} \right)}} \right\rbrack^{2}}} & (7)\end{matrix}$

By comparing the light intensities of 1^(st), 2^(nd) and 0^(th)diffraction orders, the refractive index change within the grating linescan be determined.

While specific embodiments of the present invention have been describedin the foregoing, it will be appreciated by those skilled in the artthat many equivalents, modifications, substitutions, and variations maybe made thereto without departing from the spirit and scope of theinvention as defined in the appended claims.

EXAMPLES

Extraction and Preparation of Cat Corneas

Eight corneas and eight lenses were extracted under surgical anesthesiafrom five normal, adult domestic short-hair cats (felis cattus). Allanimal procedures were conducted in accordance with the guidelines ofthe University of Rochester Committee on Animal Research, the ARVOStatement for the Use of Animals in Ophthalmic and Vision Research, andthe NIH Guide for the Care and Use of Laboratory Animals. Feline corneasand lenses were chosen because of their similarity to human corneas andlenses in terms of histological structure, molecular composition andoptical properties. See, Hughes A. The topography of vision in mammalsof contrasting life style: comparative optics and retinal organization.Handbook of Sensory Physiology, VII/5. Berlin: Springer Verlag; 1977.Also, in contrast with the problems associated with obtainingpost-mortem human eyes, using cat corneas and lenses allowed us toprecisely control post-mortem extraction time and tissue processingparameters. This was important to avoid degradation and opacification ofthe corneas and lenses prior to femtosecond laser micromachining.Extracted feline tissues were immediately drop-fixed for 10 minutes(corneas) in a solution consisting of 1% paraformaldehyde in 0.1 Mphosphate buffered saline (PBS), pH 7.4. Lenses were cut into 500 μmthick slices using a vibratome. The lens slices and whole corneas (˜500μm thick) were immersed in a mixture of 30% ethylene glycol+30% sucrosein 0.1 M PBS, pH 7.4 at 4° C. The ocular tissues were stored in thissolution at all times in order to minimize tissue swelling and loss oftransparency.

Femtosecond Laser Micromachining

For laser micromachining, the corneas were trimmed to generate small,flat pieces of tissue, averaging ˜1 cm². Each piece of cornea was thenflattened onto a clear glass slide (1×3 inches, 1 mm thick, SurgipathMedical Industries Inc., IL) with the epithelium facing up and theendothelium facing down. A glass coverslip (Corning No. 0211 ZincTitania glass) was placed on the top of each piece of cornea or lens,stabilizing it for the duration of the experiment. The ethyleneglycol/sucrose storage solution was used as mounting medium to preventor at least minimize dehydration of the cornea and lens since theseeffects are known to alter the refractive index and transparency of boththese tissues.

Example 1

Femtosecond laser micro-machining was conducted as previously describedin U.S. patent application Ser. No. 11/745,746, filed May 8, 2007 andU.S. patent application Ser. No. 11/948,298, filed Nov. 30, 2007. Thelaser source was a Kerr-lens mode-locked Ti:Sapphire laser (K-M Labs).This laser oscillator generates pulses averaging 300 mW, pulse durationof 27 fs and a 93 MHz repetition rate at 800 nm wavelength. Acontinuously variable, metallic, neutral density filter was insertedinto the optical path and used to adjust the incident laser power ontoeach cat cornea piece. The femtosecond laser pulses were focused 100 μmbelow the tissue surface using a 60×, 0.70 NA Olympus LUCPlanFLNlong-working-distance microscope objective. Because the large amount ofglass within the microscope objective induces significant chromaticdispersion into the femtosecond laser pulses, greatly broadening thefemtosecond pulse durations, we used a standard extra-cavity-prismdouble-pass configuration to compensate for the dispersion and maintainthe ultrashort pulse duration. By carefully adjusting this dispersioncompensator, we obtained nearly transform-limited 27 fs duration pulsesat the focal point of the focusing objective which were measured by acollinear autocorrelator using 3rd order surface harmonic generation(THG). During femtosecond laser micromachining, the slide containing thebiological tissue samples was mounted on a 3D scanning platformconsisting of a Physik Instrumente (PI) P-622.2CD XY scanning stage with250 μm travel range and 0.7 nm close-loop resolution, and a NewportVP-25XA linear servo Z-axis scanning stage with 25 mm travel range and100 nm resolution. An infrared CCD camera was used to monitor themicromachining process and the generation of visible plasma luminescencein real-time.

Our experiments were conducted at room temperature (˜25° C.). It tookabout 40 minutes to create a (100 μm×50 μm) grating and conduct theimmediate post-micromachining measurements. Corneal trimming andmounting did not exceed 10 minutes in duration, and the corneal tissuewas exposed to ambient air during the trimming process for at most 2minutes. Application of the Ti:Sapphire femtosecond as describedresulted in the formation of micromachined gratings having 20 to 40lines into the stroma of the corneas; each line approximately 1 μm wide,100 μm long and 5 μm apart. Refractive index changes in themicromachined regions were calculated immediately and after one month offurther storage by measuring the intensity distribution of diffractedlight when the gratings were irradiated by 632.8 nm wavelength He—Nelaser light. Because we observed no significant changes in cornea andlens transparency or thickness at the end of our micromachiningexperiments, we conclude that the described micromachining process didnot cause significant corneal or lenticular dehydration or swelling.

Example 2

Irradiation of Cat Corneas at 400 nm

Using the laser system described in Example 1 but doubling thewavelength of the light to 400 nm and making slight changes in otherlaser operating parameters one is able to dramatically increase theprocess efficiency for the formation of refractive structures in catcorneas. In one embodiment, we formed a series of refractive (line)gratings with a line spacing of 5 μm about 150 μm from the top surfaceof the cornea with Ti:Sapphire femtosecond laser above. The averagelaser power was 80 mW, the pulse duration was about 100 fs and we variedthe scan speed from 1 mm/s to 15 mm/s. At the slower speeds, i.e., 1 and2 mm/s, we observed some damage to the cornea tissue, particularly at 1mm/s scan speed. Increasing the scan speed to 5, 10 or even 15 mm/sprovided refractive structures that could only be observed using a phasecontract adjustment to the photomicrograph. The measured change in therefractive index of the focal regions for the 5, 10 and 15 mm/s scanswere 0.037, 0.03 and 0.22, respectively. FIG. 14 is a phase contrastmicrograph of a refractive structure in the form of a line gratingdescribed above that was obtained at a scan speed of 15 mm/s.

To determine the degree of physical change or potential damage (celldeath) to the corneal stroma by the micromachining process described oncan use In vivo confocal microscopy (IVCM), first described by Minsky.See, Minsky M., Memoir on inventing the confocal microscope. J. Scanning10, 128-138 (1988). IVCM allows in vivo examination of the human corneaand conjunctiva at the cellular level. IVCM is able to demonstrate thecharacteristic corneal and conjunctival anatomy in vivo at the cellularlevel. Normal corneal innervation and cell distribution, as well aschanges associated with age, contact lens wear and systemic disease suchas diabetes can be documented in vivo with this technique. IVCM has beenused to evaluate postsurgical procedures including refractive surgery,UV-crosslinking, keratoplasty and amniotic membrane transplantation toevaluate corneal wound healing. Several principles are realized inconfocal microscopes: tandem-scanning, scanning-slit and laser-scanningconfocal microscopy. Although it has high axial and transverseresolution, tandem-scanning IVCM is not able to visualize specificstructures in the cornea, such as basal epithelial cells, due to its lowlight throughput. However, it may be superior when scanning the cornealendothelium.

Another minimally invasive technique that can be used to assess physicalchanges in the corneal stroma as a result of the describedmicromachining process is the combination of reflective confocalmicroscopy with multiphoton microscopy first reported by Denk et al.,Science 1990, 73-76. See, Dong, Chen-Yuan, et al., Microscopy Researchand Technique 2008, 71, 83-85. The two imaging modalities allowdetection of complementary information from the cornea. The assessmentof epithelial cellular boundaries and nuclei, the Bowman's layer and thekeratocytes can be detected in the reflected confocal imaging mode,whereas the epithelial cell cytoplasm and the structural collagen can bedetected in the multiphoton imaging mode.

While several inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific inventive embodiments described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, inventive embodiments may be practicedotherwise than as specifically described and claimed. Inventiveembodiments of the present disclosure are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the inventive scope of the present disclosure.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

It should also be understood that, unless clearly indicated to thecontrary, in any methods claimed herein that include more than one stepor act, the order of the steps or acts of the method is not necessarilylimited to the order in which the steps or acts of the method arerecited.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

We claim:
 1. A method for providing vision correction to a patient, themethod comprising: (a) measuring a degree of vision correction needed bya patient and determining location and shape of refractive structures tobe positioned within a cornea of the patient to partially correct thepatient's vision; (b) directing and focusing femtosecond laser pulses ina spectral region between 400 nanometers (nm) to 600 nm within thecornea at an intensity high enough to change a refractive index of thecornea within a focal region, but not high enough to damage the cornea;and (c) scanning the laser pulses across a volume of the cornea to formthe refractive structures in the cornea to partially correct thepatient's vision.
 2. The method of claim 1, wherein the refractive indexof the formed refractive structure differs from the cornea tissueoutside of the focal region by 0.005 to 0.06.
 3. The method of claim 1,further comprising verifying the vision correction provided by therefractive structures.
 4. The method of claim 2, wherein the femtosecondlaser pulses have a repetition rate from 10 MHz to 300 MHz, a pulseduration of 30 fs to 200 fs, and an average power from 20 mW to 160 mW.5. The method of claim 4, wherein the femtosecond laser pulses have apulse energy from 0.01 nJ to 10 nJ.
 6. The method of claim 4, whereinthe focal region is the form of cylindrical volumes from 0.5 μm to 3 μmin diameter and 3 μm to 10 μm in length.
 7. The method of claim 2,wherein the femtosecond laser pulses have a repetition rate from 10 MHzto 300 MHz, a pulse duration less than or equal to 200 fs, and anaverage power from 20 mW to 160 mW.
 8. The method of claim 7, whereinthe femtosecond laser pulses have a pulse energy from 0.01 nJ to 10 nJ.9. The method of claim 7, wherein the focal region is the form ofcylindrical volumes from 0.5 μm to 3 μm in diameter and 3 μm to 10 μm inlength.
 10. The method of claim 1, wherein the spectral region is from400 nm to 425 nm.
 11. The method of claim 1, wherein the laser pulseshave a pulse energy between 0.1 nJ to 2 nJ.
 12. The method of claim 1,further comprising forming the refractive structure having a structuralform of at least one of a lens, a prism, a Bragg grating, a microlensarrays, a zone plate, a Fresnel lenses, and a combination thereof. 13.The method of claim 1, wherein the laser pulses have a wavelength of 400nm.
 14. The method of claim 1, wherein the laser pulses have a pulseenergy between 0.1 nJ to 10 nJ.
 15. The method of claim 1, wherein thelaser pulses have an average power from 20 mW to 160 mW.
 16. The methodof claim 1, wherein the focal region is in the form of a cylindricalvolume having a diameter between 1.0 μm to 2 μm.
 17. The method of claim1, wherein the method is performed after insertion of an intraocularlens in cataract surgery, and further comprising forming the refractivestructures to correct for aberrations resulting from the cataractsurgery or for slight misplacement of the intraocular lens.
 18. Themethod of claim 1, wherein the method is performed after ocular surgery,and further comprising forming the refractive structures to correct forlocalized optical wavefront aberrations which exist naturally or whichare induced by the ocular surgery.